The invention concerns a DC motor in general, and in particular a DC motor with an armature with several poles and with pole shoes on the armature side, such pole shoes being connected to the poles.
A known, classic DC motor consists of a rotatable, cylindrical armature, of stationary poles with pole shoes and field coils and of a cylindrical yoke. The pole shoes are widenings on the armature side, i.e. they are continuations on either side of the pole in radial cross-section, whereby the field coils are installed in the interval formed between pole shoe and yoke.
In designing a DC motor, it is important to provide for optimal magnetic ratios. In addition to the ratios of the teeth in the armature, the ratios in the yoke are equally critical and the magnetomotive force is considerably great. Assuming, for example, that the yoke thickness is 50% of the pole arc and that the yoke thickness is about 50% of the pole width, the resulting induction in the yoke will be four times higher with regards to the air gap, and the magnetomotive force will be correspondingly great.
The given space and the required number of windings in the field coils determine the ohmic resistance and therefore also the ohmic resistance losses to a great extent, just as they determine the surface of the field coils and therefore, the temperatures produced in function of the ohmic resistance losses. On the whole, this means that the application of a DC motor is essentially determined by its size, since capacity and limits of excess temperature are determined by said size within relatively narrow limits. As described above, this is based on the geometry of the poles and of the pole shoes such as they have been used in the past, as well on the resulting geometry of the field coils.
In a known DC motor with armature, poles and field coils (DE-PS No. 164 617) each pole consists of two halves which are displaced from each other by a space, in the peripheral sense, and whereby each pole half is surrounded by its own field coil. A division of a pole into several poles with the same number of windings is thus created, resulting in reduction of the electrical time constant. There are no pole shoes in this design.
For another known DC generator (GB-PS No. 325 599) partial poles, sharing a pole shoe are disclosed, whereby the pole halves with the pole shoe are approximately U-shaped in radial cross-section. This design features varying air gaps per diametrical half so that constant voltage may be attained with varying charges.
By contrast, the instant invention has as its objective to reduce the required magnetic potential different in amper-windings for a DC motor of a given size, and to reduce at the same time the ohmic resistance of the field coils while increasing the loss heat radiataing surface of the field coils.
This objective may be attained with a DC motor of the type generally described in the first paragraph above, and with the characteristics described hereinbelow.
According to one exemplary embodiment in accordance with this invention, each pole consists of two pole halves which are displaced in the peripheral sense to each other by a space. The pole halves are connected in pairs, each pair by a common pole shoe, so that each pole, in its radial cross-section, is approximately U-shaped whereby the pole halves (the legs of the U) are pointing out radially. Each of the pole halves is surrounded by a field coil of its own, so that two field coils working together are provided for each pole.
In such a design the yoke length is shortened and thereby the required magnetomotive force in the number of amper windings is reduced. By splitting the poles found in a classic DC motor and by reducing the number of windings, the ohmic resistance of the field coils is reduced, and so is the developing loss heat. This is important in continuous operation as well as in overload operation since the range of applications is basically determined by defined temperature limits.
By changing the geometric arrangement of the field coils, the surface of the coils is also increased, resulting in greater radiation of loss heat so that the developing temperatures are further lowered.
By splitting the poles and by distributing the field coils two per pole, beter use is made of the winding space and the manufacture of the coils is simplified.
Thus, the DC motor according to the invention stands out when compared with known motors in that the range of application possibilities for a given motor size is broadened, or in that the entire motor can be made smaller for given application conditions. This is necessary and desirable in a great number of cases. Yet the DC motor still remains simple in construction and inexpensive in manufacture.
According to another aspect of this invention, the distance between the two connected pole halves of the same pair should be equal to the distance to the next pole half of the next pole. This has the advantage of a symmetric arrangement of all pole parts.
According to still another aspect of this invention, it is advantageous for the number of windings of the field coils of the two pole halves to be approximately equal to the number of windings that would be used on only one field spool for a pole combining these pole halves into one single pole.
More detailed information on the invention is presented in the example of an embodiment of the invention.